Energy & Environmental Research Center, University of North Dakota,
Keywords: zero liquid discharge, evaporative cooling, brine, concentrate, power plant, data center
Summary:Zero-liquid discharge (ZLD) is often desired as a goal for industrial water treatment. However, attaining ZLD is often precluded by the near-exponential increase in treatment cost as wastewater discharge approaches zero. ZLD cooling is being explored as a way to reduce these costs by using a modified cooling tower to perform the dual duties of heat rejection and wastewater elimination. The concept is simple: unlike a conventional cooling tower, all the makeup water entering the ZLD cooling tower is evaporated to provide useful heat rejection, and the dissolved solids are precipitated to form a by-product suitable for conventional solid waste disposal. In practice, the technology is based on a cooling tower designed to use a concentrated brine working fluid instead of nearly pure water. By maintaining a terminal water chemistry, the working fluid composition remains stable, as makeup wastewater is continually added and evaporated. This approach is not without challenges. For instance, the brine working fluid has a reduced evaporation rate compared to pure water, and the materials of construction need to be compatible with high chlorides. Both factors can impact capital costs. Furthermore, the makeup wastewater must be suitable for direct contact with the atmosphere; volatile contaminants cannot be treated. However, for suitable applications, the technology has the potential to reduce the operating cost for ZLD treatment by 75% using waste heat energy, and it could fill a niche for facilities that have large cooling loads like thermal power plants, ethanol plants, and data centers. Power and ethanol plants likely produce their own wastewater that requires recycling, while a data center could recycle blowdown from its conventional cooling towers or make use of local, low-quality water sources for cooling. This presentation will introduce the technical concepts underlying ZLD cooling, including water chemistry, vapor pressure depression, and controlled precipitate growth. Data from 2021 field testing of the technology at a power plant will be reviewed. Finally, the preferred operating space for ZLD cooling will be identified and potential market applications highlighted.